An image generated by an infrared imager, such as for example a microbolometer-based infrared imager, typically includes noise. For example, the dominant source of noise may result from fixed pattern noise (FPN), particularly for uncooled microbolometer imagers. The source of FPN may be due, for example, to non-linearity of sensor elements, reflections inside the camera, temperature gradients within the system, non-linearity of the electronics, particles or water on the lens in front of the shutter, and/or a non-uniform temperature of the shutter (e.g., if not completely defocused). The source of the FPN may, at least in part, determine the appearance of the FPN in the image, with uncooled imagers typically having a higher portion of low spatial frequency FPN than cooled cameras.
In general, the FPN may have different characteristics, depending on the infrared detector, the system, and the system environment. For example, FPN may be barely noticeable or it may cause severe distortion of the image to the point that the video is no longer useful. A typical infrared imaging system may include calibration algorithms to try to minimize the effect of non-linearity and internal temperature effects. In practice, this has proven difficult and systems generally use an internal shutter that lets the system acquire an image against a uniform target to calibrate. The system may assume that any deviations from uniformity are due to FPN, with processing performed to account for the deviations (e.g., performing a non-uniformity correction (NUC)). As an example for a single shutter system with a fixed temperature, offsets in the data may be corrected (e.g., referred to as a one point correction). If the temperature of the shutter can be varied or multiple shutters at different temperatures exist, the gains can also be corrected by calculating each sensor element's response to a given temperature change (e.g., referred to as a two or multi point correction).
Often, even after performing a one or two point correction, some residual FPN will exist, because the updated NUC corrections generally only correct for non uniformity due to sources between the shutter and the detector (e.g., with the shutter between the detector and the lens). Additionally, the scene temperature is generally different from that of the shutter and the detector elements are not completely linear and, therefore, a correction made at the shutter temperature is not always appropriate for the particular scene that the system is imaging.
As a result, there is a need for improved techniques directed to infrared imaging and FPN.